Proxy 2.0 – ‘the parameter is incorrect’

When trying to add users to the ‘Proxy Authorized’ permission list (WWW), I get this error. I’ve rebooted the server, but my next step is to search through hundreds of users to see if there is an existing line in this table that it doesn’t like. Technet shows a ton of results for this error, but it’s specific to many instances, and I couldn’t find mine. Anyone ever see this?